GEOCHEMISTRY. — From γῆ, “earth,” and γυμνῆ, “chemistry.” It is the science that studies the physicochemical composition of the earth. Ever since, in an infinitely remote age, a fragment of solar matter detached itself from the incandescent mass of the star of our system to form the earth, an infinite series of chemical and physicochemical reactions has agitated the chemical elements present in solar matter; various compounds have been born and have disappeared; immense masses have changed from gaseous to liquid, then to solid, and subsequently, in part, have been remelted; the gases of the primordial atmosphere reacted with solids and liquids; the first solutions washed away or attacked the solid masses; plant and animal organisms, which appeared only very recently in this interplay of reactions, subsequently contributed to the formation of the present equilibrium. And the reactions continue: even now entire mountain chains are slowly being destroyed by the action of geological agents, and new sediments are forming within the waters, while lava masses cool and the degassing of magmas continues.
The g., then, investigates all these chemical and physicochemical reactions that constitute the present and past life of our earth—reactions that describe the genesis of our planet and its present physicochemical state and make it possible, in part, to foresee the future development of the chemical processes that will take place upon II.
G. is rightly regarded as a science of the twentieth century: it arose from the stock of geological doctrines to which it belongs, and it indissolubly links mineralogy and geology with chemistry, restoring those relationships which, during the most distinctly naturalistic period of the geo-mineralogical sciences, seemed to have grown weaker.
The theories that had guided geochemists’ investigations until recent years assumed that, during cooling, the primitive matter had differentiated by gravity, allowing an alloy of iron and nickel, nife (from Ni = nickel and Fe = iron), presumed to constitute the earth’s core or siderosphere, to sink toward the center, while depositing above it first a layer of sulfides or chalcossphere, and then the superficial layer or lithosphere, composed predominantly of silicates. The lithosphere, only 30–80 km.
thick, was divided into two portions: the sima (from Si = silicon and Mg = magnesium), formed by basic rocks, that is, rocks rich in iron and magnesium silicates, and the sial (from Si and Al = aluminum), lighter and containing aluminum silicates and free silica, consisting essentially, that is, of rocks of the granite type.
As a consequence of this division of the earth into layers, the elements too were divided into atmophilic, lithophilic, chalcophilic, and siderophilic elements, according to whether they were abundant in the atmosphere or in one of the terrestrial layers.
The hypothesis concerning the constitution of the deep layers had been reached by considering the difference between the earth’s mean density and that of surface rocks, the constitution of meteorites, and the propagation of seismic waves.
Geochemists are now turning toward a new theory, more consistent with current knowledge. According to it, the matter detached from the sun began to differentiate through cooling, immediately lost large quantities of hydrogen, helium, and other light elements, and thus became superficially enriched in heavier elements, while new streams of gaseous masses arrived at the surface, stirring the differentiated matter. Further cooling led to the condensation of the less volatile masses and to the formation of a superficial liquid shell composed of iron-magnesium silicates and possessing, on the whole, the chemical characteristics of an alkalibasic magma. It should be emphasized that, according to this theory, the degassing and increasing weight of the superficial masses caused them to sink, but only to a depth compatible with the viscosity of the matter, which, as it approached the center, became increasingly complex and compact and therefore more viscous. Consequently, beneath the liquid shell there remained a nucleus of virgin, undegassed and supercompressed solar matter.
No general differentiation by gravity or fractional crystallization therefore took place, and the present granitic masses would consequently have been formed not by differentiation of the primitive magma but by the remelting of sediments.
Indeed, above the liquid shell there was a pregeological atmosphere composed of water vapor, nitrogen, hydrofluoric, hydrochloric, and carbonic acids, and alkaline chlorides and fluorides, all perhaps at a pressure of 400 atmospheres. The further lowering of the temperature led to the consolidation of the magmatic mass, while pneumatolytic reactions produced deposits containing acidic silicates and quartz. Below the critical point of water, solutions rich in acids began to act; by attacking the superficial shell, they brought about the formation of clayey and siliceous sediments. These masses rich in silica, formed during the pneumatolytic and subsequently hydrothermal phase, accumulated by the waters in depressions and later, having sunk into zones of higher temperature, were remelted and gave rise to many of the present granites.
Investigations of the solar atmosphere show that, for every one hundred silicon atoms, there are: 150,000–300,000 hydrogen atoms, 9,000 helium, 5,000–10,000 oxygen, 1,500–3,000 carbon, 500–1,000 nitrogen, 500–1,000 fluorine, 100–1,000 magnesium, 84 iron, 82 sodium, 24 potassium, 7 manganese, 4.8 nickel, 2.9 cobalt, 2.2 chromium, 1 boron, 0.5–3 aluminum, 0.4–1.8 calcium, 0.5 vanadium, 0.3 zinc, 0.2 copper, 0.03–0.25 titanium; all the other elements are present in very small quantities. The principal constituents of solar matter, excluding those that are gaseous under our ambient conditions, are all found on earth, while, broadly speaking, the minor constituent elements of the sun are also present in the lithosphere.
The first quantitative investigations, based on numerous analyses of surface rocks, established that only eight chemical elements—oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium—participate in the formation of the superficial crust in weight percentages exceeding 1% (among them, oxygen constitutes 49.5% by weight of the crust and silicon 25.3%); four elements—hydrogen, titanium, chlorine, and phosphorus—participate in percentages between 0.1 and 0.9%, and only thirteen others—manganese, carbon, sulfur, barium,
chromium, nitrogen, fluorine, zirconium, strontium, nickel, vanadium, zinc, and copper—in percentages between 0.1 and 0.01%. The overwhelming majority of the elements is present only in traces at the surface; indeed, the sum of the percentages corresponding to the twenty-five elements cited amounts to 99.96%. If one also takes into account the volume occupied by the atoms (or ions) in the crystals of the minerals in rocks, one arrives at the unexpected result that oxygen alone occupies 91.77% of the volume of the lithosphere.
The detailed study of the distribution of the elements on our planet may be divided into three parts: the first process of distribution took place during the formation of the first high-pressure atmosphere and of the superficial molten crust, with the entrapment, at the center of the mass, of supercompressed virgin solar matter. It essentially led to the separation of helium, hydrogen, carbon, and nitrogen from the superficial matter and to the fixation of large quantities of oxygen in the silicates and oxides of the liquid mass and in water vapor.
In the 2nd process of the geochemical distribution of the elements, which took place during the consolidation of the superficial molten mass, all the minerals (almost always silicates) of the fundamental constituent elements of the magma (aluminium, iron, calcium, sodium, potassium, and magnesium) separated, while the minor constituents (the other chemical elements of the lithosphere) divided approximately into two categories: those which, by virtue of their dimensions (ionic or atomic radius), could be accommodated in the principal minerals of the rocks being formed (that is, could enter their crystals as isomorphic substitutes for the fundamental constituents), and which became diluted in the solid mass during its crystallization; and the others, the free elements, which could not enter the crystals of the common minerals and accumulated in the residues of crystallization, producing their own minerals in the pegmatitic-pneumatolytic or hydrothermal phase.
Finally, the 3rd process of the geochemical distribution of the elements takes place during the transformations that the minerals of the lithosphere have undergone and are undergoing during processes of attack and transport by geological agents, or during metamorphism.
These problems concerning the chemical constitution of the earth and the distribution of the elements are the fundamental problems of geochemistry and can only be mentioned here. Many other problems are connected with them, among which are those of biogeochemistry, that is, investigations into the cycles traversed by the chemical elements when they become part of animals and plants and of their vital processes; those concerning the distribution and transformation of radioactive substances; and those concerning the conditions under which individual minerals, and subsequently rocks, originate.
Vedi anche la serie dei lavori pubblicati da V. GOLDSCHIMID e collaboratori in Nachrichten der Gesellschaft der Wissenschaften zu Göttingen, Fachgruppe IV (Mineralogie und Geologie), 4 (1930), 6 (1930), 11 (1931), 12 (1931), 16 (1931), 18 (1932), 20 (1932), 25 (1932), 26 (1932), 27 (1932), 31 (1932), 33 (1933), 35 (1933), 36 (1933), 37 (1933), 40 (1933), 2 (1934), 4 (1934), 11 (1935), 15 (1936). F. W. Clarke, The data of Geochemistry, in United States Geological Survey Bulletin, 49 (1911).
A. Rittmann, Vulcani: attività e genesi, Napoli 1944; id., Über die Herkunft der vulkanischen Energie und die Entstehung des Sials, in Geol. Rundschau, 30 (1938), p. 52; W. Kuhn e A. Rittmann, Über den Zustand des Erdinnern und seine Entstehung aus einem homogenen Urzustand, ibid., 30 (1941), p. 215; W. Kuhn, Stoffliche Homogenität des Erdinnern, in Naturwissenschaften, 30 (1942), p. 689; H. Haberlandt, Die Bedeutung der Spurenelemente in der geochemischen Forschung,
in Monatshefte für Chemie, 77 (1946), p. 293; G. Carobbi, Proposte per un sistema cristallochimico degli ioni, in Atti e memorie dell’Accademia di Scienze, lettere ed arti di Modena, 5ª serie, 7 (1947); H. Haberlandt, Über die gesetzmässige Differentiation von Spurenelementen in Mineralien, in Tschermaks mineralogische und petrographische Mitteilungen, 1 (1948), p. 124. Guido Carobbi